The present invention relates to the field of optical communications. More particularly, the invention relates to a method and apparatus for aligning optical interconnections between printed circuit boards (PCBs), by controlling the polarization of an information carrying light beam using electrically controlled polarizer, such as a Spatial Light Modulator (SLM).
Many data communication systems, such as computers, use printed circuit boards for the integration of their electrical components. The architecture of modern electronic systems, and particularly of data processing systems, requires modular design, which is implemented by a set of PCBs, each of which is designed to fulfill specific functions. This modular design enables to more easily detect failures in the system, and to reduce production costs. Each PCB, which is frequently termed a xe2x80x9ccardxe2x80x9d, can be manufactured and tested separately, and then integrated into the system. Integration is carried out mainly by a main PCB that is often termed xe2x80x9cmotherboardxe2x80x9d (in computers, such as a PC) or xe2x80x9cbackplanexe2x80x9d (in other computerizes data communication systems), into which each card is inserted (using pins in each card, that are inserted into corresponding sockets in the backplane), and the electrical connection between them is established. The backplane provides to each card the basic required inputs, such as power lines, for operating its electrical components, and distributes a central clock signal to synchronize the operation between cards. In addition, the backplane collects digital information processed by each card via a corresponding data bus, and transfers this information to other components for further processing. The processing elements can be located on the backplane or on another card. In the example of a PC, the main processor is located at the motherboard, and memories are located on different cards. Therefore, high rate data is exchanged between each card and the motherboard, as well as between different cards, via electrical connection (data buses).
Several data communication systems require data exchange between cards in an extremely high rate. Available high speed data buses, such as a Gunning-Transceiver-Logic (GTL) (by Texas Instruments Inc., USA) bus operating at 100 MHz, allows data exchange at a rate of 100 Mb/Sec per each line. When a data rate of 2.5 Gb/Sec is required, more data lines are added to the bus and operate in parallel. However, as the demand for higher rates increases, the frequency bandwidth becomes wider, and cross-talk problems (i.e., the spectral components of the data propagating along each line overlap in frequency with those of other lines and interfere with each other) start to appear, and therefore the available bandwidth and the data rate of each line is limited. In addition, adding more lines in parallel becomes a practical solution only for very short distances.
In several data communication systems, ribbon cables are used to add more data lines externally to the printed data lines, so as to increase the rate of data exchange between cards, without exceeding bandwidth limitations due to cross-talk problems. However, using ribbon cables suffers from limitations when xe2x80x9chotxe2x80x9d replacement (i.e., replacement of cards in an essentially xe2x80x9ctransparentxe2x80x9d mode while the system is operating) of cards is required. In addition, using external ribbon cables to add data lines is also bandwidth-limited due to cross-talk problems. Therefore, backplane designs based on electrical data connections between cards are limited to a total throughput (the maximum data rate that can be processed without delaying incoming data) of 20 Gb/Sec. In addition, adding more electrical data lines to each card requires to increase the number of pins required to provide the electrical connection, which is also limited from mechanical and space aspects.
Another method for increasing the rate of data exchange between cards is to add an information-carrying optical link between the cards, in parallel to the electrical data bus. Such link is provided by modulating a light beam, such as a laser beam, with the data that should be transmitted. Using a light beam to carry the data almost removes the bandwidth limitations of a multi-line data bus. The modulated light beam is focused on the transmitting card and directed to a detector, which is normally a photo-diode detector, located at the receiving card. The laser beam is demodulated, and the data is recovered at the receiving card. However, since there are manufacturing and assembly tolerances, as well as mechanical effects caused by temperature changes of the cards and/or of the backplane, the alignment between the transmitting laser and the light receiving detector deteriorates with time, and should therefore be aligned.
Conventional techniques for the alignment of deflected light beams comprise using an array of optical devices, such as mirrors and prisms, which direct the beam back to the desired receiving point on the receiving card. xe2x80x9c60 GHz board-to-board optical interconnection using polymer optical buses in conjunction with microprism couplersxe2x80x9d, Chen et. al, Applied Physics, Letter 60 (5), February 1992 discloses a board-to-board interconnection with enhanced speed using microprisms, which eliminates the need for backplane interconnection.
xe2x80x9cHolographic optical backplane operated at 20 Gb/Sec at 1.55 xcexcmxe2x80x9d, Vincensini et al., Proceedings 21st European Conference on Optical Communications, ECOC""95 Brussels, describes a realization of a holographic optical backplane, which performs interconnections and clock distribution for six electronic boards at 1.3 and 1.55 xcexcm. High speed data transmission is provided at a data rate of 20 Gb/Sec.
xe2x80x9cHolographic coupling elements for optical bus systems based on a light-guiding optical backplanexe2x80x9d, Haumann et al., SPIE Vol. 1319, Optics in Complex Systems (1990), describes an optical backplane consisting of a light-guiding glass plate, with holographic coupling elements for coupling the light from sources into the backplane and from the backplane onto detectors with high efficiency. However, all these essentially optical techniques are costly and cumbersome. Moreover, optical alignment does not provide an optimal solution for alignment problems that are caused by changes in ambient conditions (e.g., temperature effects) or by aging of electrical components.
All the methods described above have not yet provided satisfactory solutions to the problem of aligning optical interconnections between printed circuit boards (PCBs), which overcome the drawbacks of the prior art.
It is an object of the present invention to provide a method and apparatus for aligning optical interconnections between printed circuit boards (PCBs), which employ electric control.
It is another object of the present invention to provide a method and apparatus for aligning optical interconnections between printed circuit boards (PCBs), which provide an adaptive compensation in response to varying ambient conditions.
It is a further object of the present invention to provide a method and apparatus for aligning optical interconnections between printed circuit boards (PCBs), which provide a simple compensation for mechanical tolerances.
Other objects and advantages of the invention will become apparent as the description proceeds.
The present invention is directed to a method for the alignment of optical interconnections between at least one optical data transmission point and at least one corresponding optical data receiving point. A collimated beam of data carrying light is polarized through an electrically-controlled light polarizer, such as a spatial light modulator or a liquid crystal, which comprises an array of pixels. A focused beam of data-carrying light is obtained by controlling the polarization of each pixel. More specifically, the method employs a data carrying light source, such as a laser diode for transmission from the transmission point, an optical sensor, such as a photo-diode or a photo-transistor, for receiving data carrying light at the receiving point. The sensor provides an electrical signal which is related to the energy contained in the data-carrying light. An electrically-controlled light polarizer consisting of an array of pixels, is located between the data transmission and receiving points, such that the beam of the data-carrying light is forced to pass essentially through at least one active areas of the polarizer. The beam is collimated with a lens, e.g., a Fresnel lens, or a diffractive lens, such that the collimated beam passes through a plurality of pixels. A focused beam of the data-carrying light, emitted from the light polarizer, is obtained on the receiving point, by individually controlling the polarization of each pixel, and deflecting the direction of the collimated beam when emitted from the light polarizer, until a predetermined value of the electrical signal is obtained. Additional focusing can be obtained by using a focusing lens located between the light polarizer and the receiving point. The collimated beam can be further continuously and/or adaptively aligned by determining a desired bit error rate and a threshold current and/or voltage produced by the optical sensor, which corresponds to the rate. The comparator circuitry compares the actual current and/or voltage produced by the optical sensor, to the threshold current and/or voltage, and a feedback loop controls each individual pixel according to the comparison result. The comparator generates an error signal whenever the actual current and/or voltage produced by the optical sensor is different from the threshold current and/or voltage. The error signal is fed into the feedback loop and the control signal to one or more pixels is adjusted until the error signal is reduced essentially to zero.
The invention is also directed to an apparatus for the alignment of optical interconnections between at least one optical data transmission point and at least one corresponding optical data receiving point. The apparatus comprises light emitting means, light collimating means and light-receiving means, and further comprises an electrically-controlled light polarizer consisting of an array of pixels, for focusing a collimated beam of data-carrying light. More specifically, the apparatus comprises:
a) a data-carrying light source for transmission from the transmission point;
b) an optical sensor for receiving the data carrying light at the receiving point, the sensor providing an electrical signal which is related to the energy contained in the data-carrying light;
c) an electrically-controlled light polarizer consisting of an array of pixels, located between the data transmission and receiving points, such that the beam of the data-carrying light is forced to pass essentially through at least one of the active areas of the polarizer;
d) a lens for collimating the beam, such that the collimated beam passes through a plurality of pixels; and
e) circuitry for individually controlling the polarization of each pixel, and deflecting the direction of the collimated beam when emitted from the light polarizer, until a predetermined value of the electrical signal is obtained.
In order to provide adaptive alignment, the apparatus further comprises:
a) a comparator circuitry for comparing the actual current and/or voltage produced by the optical sensor, to a threshold current and/or voltage;
b) a feedback loop for controlling each individual pixel according to the comparison result;
c) circuitry for generating an error signal by the comparator, whenever the actual current and/or voltage produced by the optical sensor is different from the threshold current and/or voltage;
d) electrical connection for feeding the error signal into the feedback loop; and
e) circuitry for adjusting the control signal to one or more pixels until the error signal is reduced essentially to zero.
The invention is also directed to a data communication system, which comprises means for the alignment of optical interconnections between at least one optical data transmission point and at least one corresponding optical data receiving point. The means employed in such system comprises light emitting means, light collimating means and light-receiving means, and further comprises an electrically-controlled light polarizer consisting of an array of pixels, for focusing a collimated beam of data-carrying light. The polarizer may be controlled adaptively, to compensate for dynamic and/or time dependent deflections of the collimated beam from focus.
The invention is further directed to a data communication system for the alignment of optical interconnections between at least two optical data transmission points and at least two corresponding optical data receiving points, which comprising:
a) at least two data-carrying light sources for transmission from different transmission point;
b) at least two optical sensor for receiving the data carrying light at the corresponding receiving points, each of the sensors providing an electrical signal which is related to the energy contained in its destined data-carrying light;
c) an electrically-controlled light polarizer consisting of an array of pixels, located between the at least two data transmission points and their corresponding at least two data receiving points, such that different beams of each data-carrying light are forced to pass essentially through different active areas of the polarizer;
d) at least one lens for collimating the different beams, such that each collimated beam passes through a different group of pixels of the polarizer; and
e) circuitry for individually controlling the polarization of each pixel in each group of pixels, and deflecting the direction of each collimated beam when emitted from its corresponding active area of the light polarizer, until a predetermined value of the electrical signal is obtained at each receiving point.